This invention relates to an improved method and system for controlling a pot welding process in real time which is capable of dealing with the numerous process parameter variations and non-linearities present in this welding process.
Resistance spot welding has long been used as a method of joining sheet metal. It is known that in order to guarantee the quality of the spot weld as the electrodes wear and in the presence of part fit up anomalies and machine variation, a real time power control is required. The weld nugget is formed between metal workpiece sheets which are held together with a suitable welding force as shown in FIG. 1. After good contact has been established between the workpiece and the electrodes a large current is generated through the workpiece by a power supply. Heat is produced in the workpiece due to metal contact resistance and the current generated between the electrodes. The quality of the weld is influenced by the electrode condition, the metal to metal contact resistance, and the material surface condition. Each of these variables affects the heat produced at the workpiece by changing the effective resistance at the workpiece. As the effective resistance changes it is necessary to increase or decrease the current magnitude in order to maintain the power to the workpiece at a level which will melt the metal and produce a good weld. The machine calibration (heat command versus current) is also another source of heat error which has to be compensated. The resistance, in addition to changing from weld to weld, varies with time during the weld requiring that the power control be regulated in real time. In order to produce acceptable welds with the present machines it is necessary to measure weld quality periodically with test coupons and adjust the machine heat bias to insure that the constant heat applied produces acceptable welds. These tests are labor intensive and to not guarantee than good welds are produced all the time. The use of adaptive controls will not only ensure the quality of each weld but will also eliminate the requirement for test coupons thus increasing productivity and reducing costs.
Resistance spot welding has received considerable attention relative tocomputer control and instrumentation. In U.S. Pat. No. 4,596,917, "Resistance Spot Welder Process Monitor", sensors on the machine measure welding input variables and a simplified analytical model of the spot welding process is the reference for the weld quality logic required for monitoring the process. U.S. Pat. No. 4,447,700, "Resistance Spot Welder Adaptive Control", discloses monitoring the spot welding machine with a microcomputer which provides a real time data acquisition of the various welding parameters. The control uses thermal expansion and its first derivative to control the welding power and cooling rate as a means of controlling the quality of the nugget. In each of these prior inventions significant improvements were made in monitoring the quality of the weld. There is no specific definition of a real time adaptive control which regulates the heat input to the spot weld.